Evolution mechanism of coal–oxygen adsorption in underground coal fires: from physisorption to chemisorption

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There is a consensus that coal–oxygen adsorption is the crucial first step in coal spontaneous combustion (CSC), which includes physisorption and chemisorption. However, owing to the absence of theoretical models, the scientific question how physisorption gradually transits to chemisorption is not clear, leading to difficulties in reasonable explanation for the evolution mechanism behind the transition stage which is found in previous research. To unravel this veil, the concepts of associative adsorption and dissociative adsorption have been imported into coal–oxygen adsorption. Meanwhile, the evolution mechanism model has been theoretically established and verified by programmed heating experiments and oxygen physico-chemical adsorption–desorption experiment. Both experiments respectively explored the essence of coal–oxygen adsorption from the perspectives of gas products and reactants. It is determined that physisorption is followed by associative adsorption and dissociative adsorption. In the atmosphere with oxygen concentration of 20.9%, associative adsorption has occurred before 25°C, and dissociative adsorption has occurred before 60°C. The theoretical model sheds light on the mechanism behind the evolution process from physisorption to chemisorption and reveals sequence of different types of adsorptions: physisorption, associative adsorption, dissociative adsorption. The research findings are anticipated to offer theoretical backing for coal mine fire hazard prevention and effectively resistant materials advancement.